1. Field of the Invention
The present invention relates to an image signal compressing and coding apparatus, and, more particularly, to an image signal compressing and coding apparatus which is adapted for use in an image processing apparatus that uses digital moving images.
2. Description of the Related Art
Recently, various techniques for compressing and coding moving images at high efficiency have been studied and developed for moving image processing apparatuses which process moving image signals typified by color TV signals.
The conventional coding schemes involving the compression of moving images typically use a frame (I) for intraframe coding and a frame (P) for interframe differential (compression) coding (hereinafter referred to as "interframe coding frame") in mixture along the time axis, as shown in FIG. 5. The interframe coding is a very effective compressing scheme when the correlation between frames is high. That is, intraframe coding frames are inserted between a plurality of interframe coding frames to improve the compressing efficiency and minimize the disturbance of images even if some images are dropped off.
A conventional moving image processing apparatus will now be described with reference to FIG. 6.
In FIG. 6, an image signal in a raster signal (not shown), which is output from an image input device, is converted into a block scan signal by a raster/block converter 1. The processing carried out at the subsequent stages of this raster/block converter 1 differs between the intraframe coding frame and the interframe coding frame. To begin with, the processing for the intraframe coding frame will be discussed.
The image signal, which has been converted into a block scan signal, is input to a DCT (Discrete Cosine Transform) circuit 5 via a data selector 4 to be converted into a DCT coefficient. This DCT coefficient is subjected to quantization and Hoffman coding by an encoder 6 based on a previously prepared quantization table, and the resultant data is stored in a code memory 7. One frame of data stored in the code memory 7 is read out as a compressed file to an unillustrated, external storage device (hard disk, tape streamer, magneto-optical storage device, or the like) and is also input to a decoder 8.
This decoder 8 performs Hoffman decoding and inverse quantization to generate a DCT coefficient. The DCT coefficient is input to an IDCT (Inverse Discrete Cosine Transform) circuit 9 to be converted into image data. The converted image data is sent to an adder 11, which reconstructs the original image from the differential frame for an interframe coding frame. In the case of the intraframe coding frame, "0" is selected by a data selector 10 so that the output of the adder 11 does not change.
The output of the adder 11 is written in a frame memory 12 and is also input to a data selector 2 through which the output is sent to a differentiating circuit 3.
The processing for the interframe coding frame will now be described. The image signal, which has been converted into a block scan signal by the raster/block converter 1, is input to the differentiating circuit 3 to obtain the difference between this block scan signal and the image signal from the data selector 2, which has been compressed or expanded by the intraframe coding in the above-described manner. The resultant image data is then sent to the DCT circuit 5 via the data selector 4 to be converted into a DCT coefficient. The DCT coefficient is subjected to quantization and Hoffman coding by the encoder 6 based on the previously prepared quantization table and the resultant data is stored in the code memory 7.
One frame of data stored in the code memory 7 is read out to the unillustrated, external storage device and is input to the decoder 8 at the same time. This decoder 8 performs Hoffman decoding and inverse quantization to generate a DCT coefficient. The DCT coefficient is input to the IDCT circuit 9 to be converted into image data. The converted differential image data is sent to the adder 11, which adds this data and the image data of the previous frame together to reconstruct the original image. The image data of the previous frame from the frame memory 12 is selected by the data selector 10. The output of the adder 11 is written into the frame memory 12 and is input to the data selector 2 simultaneously. The subsequent frames will be processed in the above-described procedures.
The data selector 2 selects "0" only when the first frame is coded, and selects the output of the adder 11 thereafter.
The decoding procedures will now be discussed. Coded data is read from the unillustrated external storage device and is buffered by the code memory 7. The data is then input to the decoder 8 to be subjected to Hoffman decoding and inverse quantization, so that a DCT coefficient is generated. This DCT coefficient is input to the IDCT circuit 9 to be converted to image data, which is then sent to the adder 11.
In the case for the intraframe coding frame, "0" is selected by the data selector 10, so that the output of the adder 11 does not change. For the interframe coding frame, image data of the previous frame stored in the frame memory 12 is selected by the data selector 10 and is added to the differential image data by the adder 11. The output of the adder 11 is converted into a raster signal by a block/raster converter 13 and is output therefrom as an image signal.
The data selector 4 selects the output of the raster/block converter 1 only when the first frame is coded, and selects the output of the differentiating circuit 3 thereafter.
Since the prior art performs image expansion in the image compressing process and uses a differential frame obtained from the expanded image data, the image expanding process should also be executed at the time the image compressing process is carried out.
As the compressing and expanding processes in the prior art are all executed with block scan signals, an FIR (Finite Impulse Response) filter, which serves as a low-pass filter whose filtering amount changes substantially in accordance with the spatial frequency band to eliminate deformation, cannot be used at the time of effecting the filtering to eliminate the distortion of the boundary between blocks as disclosed in U.S. patent application Ser. No. 813,798 (replaced with contination application Ser. No. 08/238,983) that was filed by the same assignee as that of this application.
Further, because the filtering algorithm for moving images differs from that for still picture images, the prior art requires different circuits to perform compression/expansion for moving images and compression/expansion for still picture images.
In addition, it is difficult in the prior art to monitor the statuses of the compression of moving images or still picture images, the filtering coefficients, and the compression-oriented deterioration of images.